Just in case it helps: as you are getting a response from the remote server (being the HTTP 408 error), I’d not look for DNS problems: to be able to connect to that server you clearly got an IP address for the domain name you’re trying to access, so DNS worked, I’d say.
Also, as Docker uses HTTP 429 Too Many Requests when running into rate limits, I’d not first look for rate limit problems when instead seeing a 408 Request Timeout.
Is that the actual command you’re using, with helloworld instead of hello-world? I assume it should not matter for the 408, but what if you try with an image that actually exists?
Aside, searching for 408 on this very forum yields:
I’d assume that for such solution not only Docker would give you problems, but I’ve no idea.
Before diving into that: maybe you could repeat your curl tests with a long value for INVALIDTOKEN (at least the same length as a valid token). And repeat it using the valid token on another computer, preferably on the same network/IP address.